PROJECT SUMMARY/ABSTRACT Widely used as a dietary supplement, the antioxidant trace element selenium (Se) is essential for human health. Se is known for its role in curbing oxidative stress and in the regulation of thyroid function and male fertility. Past studies have associated altered selenoprotein expression with increased risk for metabolic syndrome (MetS). Whole-body knockout (KO) of the intracellular Sec decomposition enzyme Sec lyase (Scly) in mice increases susceptibility to MetS and diet-induced obesity (DIO), accompanied by decreased selenoprotein expression in the hypothalamus, a key mediator of energy homeostasis. Surprisingly, mice with targeted Scly KO in agouti-related peptide (Agrp)-positive neurons of the hypothalamus are protected from DIO and leptin resistance, an effect that may be influenced by thyroid hormone function in males. Mice with Agrp neuron-specific deletion of the selenocysteine-tRNA (Trsp), which is essential for selenoprotein expression, display metabolic effects similar to those observed in Scly-Agrp KO mice. These results are specific to female mice, however, as male Trsp-Agrp KO mice show no significant differences from controls. The overall goal of this proposal is to elucidate the mechanisms that underlie the metabolic phenotype of Scly-Agrp KO mice and the sex differences observed in Trsp-Agrp KO mice. The central hypothesis is that Agrp neurons depend on Se utilization to mediate high-fat diet-induced changes via sex-specific mechanisms. Specifically, this proposal investigates the possibility that loss of Scly protects Agrp neurons from developing early leptin resistance to prevent downstream hypothalamic leptin resistance and limit weight gain by maintaining brown adipose tissue thermogenesis. This project will also test the hypothesis that loss of Trsp causes progressive neurodegeneration of Agrp neurons, which impacts sex-specific differences in hypothalamic neurogenic mechanisms to result in the metabolic phenotypes observed. These hypotheses will be tested using these mouse models and a combination of in vivo phenotyping, tissue analysis, and ex vivo electrophysiology on live hypothalamic brain slices. This fellowship will take place under the guidance of an experienced mentor and pioneer in the Se field, and an accomplished co-mentor with an extensive background in neuroscience. The proposed individualized training plan includes specific educational and career development activities, and benefits from the expertise of a collaborative community of scientists implementing an intensive team mentoring program. It is anticipated that the experiments proposed herein will shed light on the important roles of Se utilization and selenoproteins in metabolic disease pathology, provide new information on the interplay between the central nervous system and whole-body energy metabolism, and may potentially identify key targets of interest for preventative strategies or therapeutic treatments for metabolic disorders.